The present invention relates to frozen tofu products, and their production. Tofu (soybean curd) products [e.g., tofu, atsuage (thick fried tofu), etc.] have heretofore been said to be difficult to store for a long period of time, and have also been said to be difficult to subject to freezing storage. However, even when the tofu products of the present invention are frozen, and stored for a prescribe period of time, they are scarcely denatured by freezing. Further, when they are thawed and cooked for eating, they have excellent taste and cooking applicability, and are smooth.
As is generally known, tofu is produced by soaking soybeans in water for a certain period of time, grinding the soybeans, adding water thereto, heating the resultant slurry or puree and separating okara (soy pulp or tofu waste) to obtain hot soybean milk. To this soybean milk, a solidifying agent (coagulant) is added to obtain tofu. When this tofu is frozen, spongy tissue or layered tissue such as frozen tofu (k{overscore (o)}ya tofu) is formed, thereby spoiling smooth mouthfeel before freezing. Then, various examinations are carried out to improve this.
For example, JP 6-217729 A and JP 6-269257 A propose processes for producing tofu having freeze resistance, wherein transglutaminase, starch and the like are added to soybean milk. However, when tofu is produced by these processes in a large scale, it is very difficult to control a coagulation reaction because of the increase in viscosity of soybean milk, and there are problems in the production of tofu having good tissue.
In addition, JP 9-182571 A proposes a process for producing frozen tofu, wherein starch and the like are added to soybean milk concentrated to 14% to 20%. However, unless a solidifying agent is added to soybean milk at a temperature of 30xc2x0 C. or lower, taste and mouthfeel of tofu cannot be improved because of the increase in viscosity. Then, at present, an industrial process which can stably produce tofu having excellent freeze resistance is not yet established.
Further, after thawing frozen tofu stored for a prescribed period of time, cooking applicability of thawed tofu is spoiled and, when a lot of tofu are cooked in a large batch, they are crumbled or broken. A process for producing tofu maintaining excellent cooking applicability after thawing and cooking is not yet established.
As described hereinabove, a method for efficiently improving freeze resistance of tofu does not yet become popular. The present inventors have studied freeze resistance of tofu. Thus, the present invention is to provide a process for industrially producing frozen tofu products which are not denatured even by freezing, and have the same mouthfeel and taste as those of fresh tofu even after thawing, and are excellent in cooking applicability.
The present invention is tofu products having freeze resistance obtainable by adding one or more members selected from the group consisting of sugars, starch and transglutaminase to soybean milk having low viscosity and a solids content of at least 10%; adding a solidifying agent to prepare tofu; and then freezing. The present invention also provides a process for producing the tofu products.
More specifically, in order to obtain soybean milk having low viscosity, concentrated soybean milk having low viscosity and a high solids content can be produced by extracting soybean milk from uncooked soybean slurry or puree prepared from whole soybeans, and concentrating the soybean milk so that the solids content after heating becomes 10% to 16%. To this concentrated soybean milk, 0.1 to 5% by weight of the sugars or starch, or 0.01 to 2% by weight of transglutaminase, and the solidifying agent are added at the temperature of the concentrated soybean milk of 60xc2x0 C. or higher to prepare tofu. The tofu is heated to elevate its temperature stepwise to obtain tofu having elasticity. The tofu thus obtained has freeze resistance and is excellent in cooking applicability.
Hereinafter, the present invention will be illustrated in detail. In the process for producing tofu products having excellent freeze resistance of the present invention, it is better to heat and concentrate uncooked soybean milk extracted from fresh soybean slurry or puree to obtain concentrated soybean milk having low viscosity and a high solids content. Further, in order to obtain tofu having excellent elasticity, preferably, the additive and the solidifying agent are added to the above concentrated soybean milk wormed to 60xc2x0 C. or higher, followed by further heating the mixture stepwise (e.g., two steps) to elevate the temperature.
In a conventional process for producing tofu, generally, soybean milk is extracted from cooked soybean slurry or puree. That is, soaked soybeans are ground together with water to obtain soybean slurry or puree. The soybean slurry or puree is heated with live steam, followed by separating okara therefrom to obtain soybean milk. When soybean slurry or puree is treated by heating in this manner, viscosity of the soybean milk is increased, which adversely affects on the subsequent coagulation reaction.
In contrast, in the present invention, soybean milk is extracted from uncooked soybean slurry or puree. That is, soaked soybeans are ground together with water to obtain soybean slurry or puree. Okara is separated from the soybean slurry or puree to obtain uncooked soybean milk. This is concentrated by heating (at 90xc2x0 C. or higher for 5 minutes or more). The resultant concentrated soybean milk has low viscosity, while it has a high solids content. It has been confirmed that good tofu is obtained by the subsequent coagulation reaction of this concentrated soybean milk. The term xe2x80x9clow viscosityxe2x80x9d used herein means that viscosity of soybean milk at 10xc2x0 C. is not higher than 100 mPaxc2x7s or lower, preferably, 50 mPaxc2x7s or lower.
The concentration method is not limited to a specific one. For example, good concentrated soybean milk can efficiently be obtained by heat concentration under reduced pressure with a centrifugal thin membrane because excessive heating of soybean milk can be avoided. In the present invention, preferably, the solids content of soybean milk is adjusted to 10 to 16% by weight. When the solids content is less than 10% by weight, mouthfeel of the resultant tofu becomes inferior, and tends to be denatured after freezing. When the solids content is over 16% by weight, viscosity of soybean milk is rapidly increased and it is difficult to subject the soybean milk to an appropriate coagulation reaction. Therefore, it is necessary to adjust the concentration of soybean milk within the above range.
Next, to the concentrated soybean milk obtained by the above method, one or more members selected from the group consisting of sugars, starch and transglutaminase are added to prevent freeze denaturation.
Examples of sugars include sucrose, fructose, xylose, sorbitol and the like, with oligosaccharides having low sweetness, sugar alcohol such as erythritol, maltitol, etc., trehalose, and the like being preferred.
Starch may be cereal starch such as corn starch, wheat flour starch, etc., and their processed products such as xcex1-starch, oxidized starch, etc., with starch which has freeze resistance without syneresis being preferred.
Transglutaminase is an enzyme normally obtained by culturing a microorganism with starch, etc. as a fermentative raw material, and is an outstanding enzyme for crosslinking proteins in food to produce a close and smooth texture. In the present invention, a transglutaminase preparation containing an edible material such as starch, etc. may be used.
The temperature of the soybean milk at which the solidifying agent are added the concentrated soybean milk is preferably 60xc2x0 C. or higher. Although bad taste of soybean milk can be somewhat improved by the above concentration, if the temperature is lower than 60xc2x0 C., sufficient improvement of taste is scarcely expected and the resultant product has less specific taste of tofu and less elasticity. This may be due to a too slow reaction between proteins in soybean milk and the solidifying agent. Further, in case of addition of starch, when the temperature is higher than 70xc2x0 C., viscosity of soybean milk is rapidly increased due to gelatinization of starch and it is difficult to subject soybean milk to an appropriate coagulation reaction. Then, attention should be given to the temperature of soybean milk.
The solidifying agent to be used in the present invention includes a bittern normally used in the tofu production, calcium salts such as calcium sulfate, etc., magnesium salts such as magnesium chloride, glucono delta-lactone and the like, with calcium sulfate whose a coagulation reaction is gentle being preferred.
Next, the tofu produced above is aged at 60 to 70xc2x0 C. for 10 to 60 minutes so that transglutaminase acts on the tofu, and the coagulation reaction with the solidifying agent becomes gentle to facilitate the stable production of the tofu. Then, the tofu is heated to elevate its temperature stepwise to inactivate transglutaminase and to promote the coagulation reaction in the tofu to provide elasticity. When the latter heating is carried out in combination with the above heating at 60 to 70xc2x0 C. by 2 steps, the tofu is heated at 80 to 90xc2x0 C. for 10 to 60 minutes. If the final heating is carried out at lower than 80xc2x0 C., transglutaminase is insufficiently inactivated and elasticity of the tofu is also insufficient. If the heating is carried out at higher than 90xc2x0 C., the tofu tissue is spongy and its mouthfeel is inferior.
Then, the tofu is cut into a prescribed size. After cooling to 40xc2x0 C. or lower, the tofu is frozen with a quick freezer. Alternatively, after cutting into a prescribed size, the tofu may be fried to prepare so-called xe2x80x9catsuagexe2x80x9d and then quickly frozen. The frozen tofu products thus obtained can keep the mouthfeel of tofu for more than 6 months.
In addition, cooking applicability such as a lot of tofu can be cooked in a large batch without crumbling or breaking is desired in a market of tofu for business use, where there are much opportunity that a lot of tofu are cooked in a large batch such as meals for schoolchildren, workers, etc.